1. Field of the Invention
The present invention relates to a control method for supplying an inert gas into a storage container having a heater, the control method being employed when ingots or metal materials cast in bar shapes are melted and retained in the storage container, and the molten material is fed to a metal molding apparatus to produce metal products.
2. Description of the Related Art
Conventionally, a technique is known for supplying an inert gas into a melting container in order to prevent oxidation of the melt retained in the melting container (for example, see Japanese Patent Application Laid-Open No. 2001-311643 which corresponds to U.S. Patent No. 6,487,905). Means for supplying an inert gas is also known which serves to reduce the consumption of the inert gas as well as to address drawbacks of excessive throttling or valve closing with a proportional flow regulator (for example, see Japanese Patent Application Laid-Open No. 2002-361386). In the supplying means, an inert gas tank is connected to the proportional flow regulator via a conduit to supply the inert gas to both a hopper and a nozzle at a cylinder tip through the proportional flow regulator. Then, a cylinder temperature determined by a temperature sensor is compared with a temperature derived from a temperature table having been set in advance in a stepwise manner, thereby controlling the degree of opening of the proportional flow regulator. The supplying means can provide automatic variable control to the amount of the inert gas being supplied in accordance with the cylinder temperature.
The above control method for supplying the inert gas provides control to the degree of opening of the proportional flow regulator based on the comparison between the cylinder temperature and a temperature derived from the temperature table set in advance. This control method can supply the inert gas corresponding to the cylinder temperature, thereby saving the inert gas and solving the drawbacks of poor flow regulation. However, the supply control carried out based on the comparison between the cylinder temperature and a temperature derived from the temperature table is not intended as a countermeasure against a gas supply shortage caused by its depletion while the gas-supply operation is continued. Accordingly, the shortage of the inert gas does not allow the metal material to be melted and retained in an atmosphere of the inert gas. It is thus difficult to prevent the occurrence of a large amount of impurities resulting from oxidation of the molten metal material or to prevent the material from catching fire due to the oxidation.